Well, for sure if I was in the parks from 9:00 a.m to midnight, I would find walking anything longer than 100 feet very difficult!
CBR is large, but we find it very doable and well laid out.
If you are concerned about size, preferred might be a good idea. We really enjoyed our location in Jamaica 43, it took about five minutes to get over to OPR from our room at a leisurely pace. Our first stay at CBR was in Barbados and that walk generally took around 5 minutes as well (I think we were in bldg 16).
Pretty much any standard (as in non pirate room) room will be less than a ten minute walk to the main area. And bear in mind each village has its own pool and bus stop. There is also an internal bus that I never saw anyone get on and the bellman who took us to our room said it was generally easier and quicker to just walk. Now if you have already been walking all day in the parks and have young kids, the walking would probably take longer. The food court does close at 11:00 so for really late nights, its best not to even bother going over to the food court.
Most people will say that Aruba and Jamaica are good locations since they are just across the bridge, although within each of those sections, there are several buildings, some that are not as close as others.
I personally would not pay for a preferred room at CBR, however that is really subjective. It might be worth it for the peace of mind alone! We have friends that stayed at CBR in October with young kids (their ds is 6) and they got a preferred room and while they didn't think it was a waste, after staying there and seeing the size and scope, they said they would just book standard next time.
